The present invention relates generally to a color image reading apparatus and, more particularly is directed to a color image reading apparatus for use in color scanners, color facsimiles, etc., that offer a high accuracy reading of the color image information while shutting out extraneous light therefrom When the surface of an original is scanned, by application of a color separation element consisting a blazed diffraction grating and a detection means including three line sensors.
Hitherto several varieties of readers have been introduced that digitally read the color image information of an original by utilizing the output signal of line sensors such as CCDs on which the color image information is formed through an optical system.
FIG. 1 shows a schematic diagram of a conventional color image reader.
FIG. 1 shows that a light beam from the color image on an original surface 1 is condensed by an image-formation lens 15. When the image is to be formed on the surface of the line sensors, explained later, a 3P-prism 16 is inserted in the said light beam path. After color separation of the beam into, for instance, the three colors red R, green G and blue B, the light is introduced to the surface of the line sensors 17, 18 and 19 which are each comprised of CCDs or the like. Then, the color images formed on the surface of the line sensors 17, 18 and 19 are read separately for each color by means of line scanning.
FIG. 2 shows the essential part of a color image reader disclosed in Japanese Patent Application Laid-Open No. 62-234106.
FIG. 2 shows that the light beam from the color image on the original surface 1 is condensed by an image-formation lens 28. When the image is to be formed on the surface of the line sensors, explained later, the light beam is separated into three beams corresponding to the three colors by means of beam splitters 29 and 30 for color separation, that are equipped with a film that selectively allows transmittance of two colors. Then, the three color beams that make up the color image are directed to three line sensors 31a, 31b and 31c installed on the same substrate, the so-called monolithic 3-line sensor 31, and the image is formed on the surface of each of the line sensors.
Then the color images are line scanned separately for each color.
The U.S. Pat. No. 4,277,138 (corresponding to DE No. 2645075) introduces a color image detector. In this case, a blazed diffraction grating is used as the optical element for color separation. The incident color image information separated into colors are thereby detected.
The color image reader shown in FIG. 1 becomes expensive and generally complicated since it requires the application of three independent line sensors, and high accuracy is required. Further, the difficulties involved in manufacturing the 3P-prism must be considered. Another problem is that adjustment of the system is bothersome since it is necessary to make three independent adjustments of the condensed beams sent to each line sensor.
In the case of the color image reader shown in FIG. 2, if the plate thickness of the beam splitters 29 and 30 is indicated as t, the distance, between each line of the line sensor becomes 2..sqroot.2f. Line sensors produced now come preferably with a distance of generally 0.1-0.2 mm between each line. This means that the plate thickness t of the beam splitters (29) and (30) must be about 35-70 .mu.m.
In general, it is extremely difficult to manufacture beam splitters that maintain an optical plane surface character when the plate is as thin as in this example. Applying beam splitters as thin as these, poses a problem as the optical performance of the color image formed on the surface of the line sensors deteriorates.
U.S. Pat. No. 4,277,138 (corresponding to DE No. 2645075), which is a color image reader where a dichroic mirror is substituted by a blazed diffraction grating, poses a problem since it only treats one beam from each point of the surface of the subject. If, for instance, a reflective original is being read, extraneous light not belonging to the optical axis of the beam may pass through the blazed diffraction grating and the incident light reaching the line sensors will contain noise light of a wrong color composition.
The present invention is intended to provide a color image reader that can digitally read with high accuracy a color image by utilizing for instance the three color beams R, G and B. When reading a color image by color separation utilizing one-dimensional blazed diffraction grating, a slit, that satisfies predetermined conditions, positioned between an original surface and a projection optical system, effectively prevents noise light, in the form of extraneous light not belonging to the optical axis of the beam and created by the diffraction, from reaching the line sensors on which the image is formed.
In the color image reader according to the present invention, the color image on the surface of an original is illuminated by means of an illumination device. Said color image is projected by the projection optical system to the surface of the detection device where three line sensors are arranged parallel on an identical substrate. When said color image is read by said detection device, the one-dimensional blazed diffraction grating, that which separates the light beam from said projection optical system into three beams of separate colors in the direction normal to the arrangement direction of the picture elements of the line sensor's and also leads the beams to three respective line sensors, is positioned behind said projection optical system. Also, a slit having an aperture which extends longer in the arrangement direction of the picture elements of the line sensors is positioned in the optical path from the original surface to said projection optical system. This arrangement prevents noise light in the form of extraneous light caused by diffraction.
To effectively prevent noise light, the prescribed elements should meet the conditions that EQU L1'/2&lt;L2
where L1' is the length of penumbra on the surface of said detecting means in the shorter direction defined by said projection optical system and the slit, and L2 is the shortest interval between the sensors in the direction normal to the array direction of said three line sensors.
In addition, in the present invention said slit has a transparent aperture formed by the application of etchings on the base surface that absorbs infrared light, and the slit is installed in the way that allows it to move in the array direction of the line sensors.